How to point a sun lamp using a vector in Python?
There is indeed a straightforward python method using mathutils.

from mathutils import Vector
direction = Vector(your_tuple)
your_object.rotation_mode = 'QUATERNION'
your_object.rotation_quaternion = direction.to_track_quat('Z','Y')


You can see the doc here for more details.

Edit: As the sun direction is negative on the z axis, you may have to change 'Z' to '-Z'